Improve Support for @memberof inference with ES6 modules

A common pattern in modern js web development is to have several named utility modules with named and default exports.

It would be ideal if `documentation.js` was smart enough to infer the membership status of private and exported methods that were documented inside the module.

I've set up a demonstration to illustrate what I'm after here:

https://github.com/ajhyndman/documentation-js-demo/commit/9ef86935e7682fb0d3217c534c53b1a86a49c086